A series of experiments are proposed for investigating the manner in which infants and young children perceive snythetic and natural speech sounds as well as nonspeech stimuli that match the complex temporal and frequency relations present in speech. Discrimination and categorization data will be obtained from 6- to 12-months-old infants using an operant headturning procedure. Discrimination, categorization, and labeling data will be obtained from 2- to 4-year-olds using a two-alternative pointing procedure. Studies of infants will focus on the discrimination of foreign speech contrasts, trading relations, categorization of stop consonant place of articulation and vowel category in CV syllables, and the extraction of phonetic features. Studies of young children will focus on the discrimination of foreign speech contrasts, the discrimination and labeling of vowels, vowel normalization and reduction, context effects, trading relations, feature extraction, and the discrimination and labeling of place of articulation in stops, VOT, and TOT. In addition to these studies of speech perception, experiments with 6- to 12-months-olds will address several aspects of developmental psychoacoustics, including thresholds for high frequency tones, frequency discrimination, discrimination of frequency modulation, and psychophysical tuning curves. These psychoacoustic studies will not only establish norms for this age range but also uncover any sensory constraints on speech discrimination. The overall goal of this program of research is to evaluate the mechanisms underlying speech perception in infancy and early childhood. The role of early linguistic experience, particularly in the early stages of language production, will be examined, as well as the onset age for a phonetic mode of analysis.